The present invention relates to optical networking and more particularly to verifying optical connections.
Optical cross-connects (OXCs) are an important building block in realizing the goal of all-optical networks. By definition, optical cross-connects do not involve conversion to electrical signals and regeneration of optical signals. They provide extremely high throughput to accommodate mass broadband access to multi-media applications. Optical cross-connects are highly advantageous in DWDM (dense wave division multiplexing) systems where multiple wavelengths share the same fiber. Furthermore, optical cross-connects readily accommodate mixing of optical signals that are disparate in their framing structures and/or data rates.
An optical cross-connect system includes a plurality of input ports, a plurality of output ports, and a switching fabric capable of connecting a selected input port to a selected output port. The path between the selected input port and the selected output port is said to be transparent because there is no conversion to electrical signals. The switching fabric may incorporate micro-electro-mechanical system (MEMS) technology. The MEMS integrated circuit includes rotatable mirrors and other mechanical and optical components.
Optical cross-connects are key components in the service provider networks in which they find application. As such, extremely high reliability is demanded. Correct operation should be monitored continuously and faults should be addressed immediately. One aspect of the continuous monitoring of optical cross-connect performance is verification that the correct input ports and output ports are in fact coupled together. Connection verification is an important requirement in providing what is now known as xe2x80x9ccarrier class reliability.xe2x80x9d
Various approaches to the problem of verifying optical cross-connections have been studied. One possible approach is to examine marker data on the optical signals at the two ports that are supposed to be connected to verify that this marker data is in fact the same. For example, if the optical cross-connect is relaying SONET data, one can make use of the path tracing bytes in the SONET frame structure for this purpose.
Insertion of the necessary marker, however, requires optical-electrical-optical (O/E/O) regeneration at both the input and output port to insert and extract the marker. This, however, goes against one of the principle reasons for using the optical cross-connect, namely, the removal of the need for regeneration. Furthermore, this technique cannot be readily applied to DWDM signals which carry multiple payload signals. This technique also cannot perform verification where the optical cross-connect carries multiple signals with disparate data formats.
Another solution is to modulate the signals incident at the input ports to the cross-connect with a pilot tone and check pilot tone content at the output port to verify connectivity. The pilot tone could be inserted at client equipment that originates the optical signal or such modulation could be provided as a part of the cross-connect using opto-electronic devices such as LiNbO3 optical modulators. Again, there are numerous drawbacks. If the modulation function is performed at the client side, it will be difficult to coordinate appropriate pilot tone content among multiple clients. If the opto-electronic equipment is included at the cross-connect, the expense of the cross-connect will increase dramatically. Another problem is that applying the pilot tone within the cross-connect will modify characteristics of the optical channel, possibly degrading the optical signal to an unacceptable degree. Such degradation is particularly disadvantageous if the cross-connect path is part of a long span without regeneration.
It is also possible to add a tag optical signal at a different wavelength at the selected input port and then verify the presence of the tag optical signal at the output port. The tag signal can then be removed at the output port. As with the pilot signal technique, many additional lasers and detectors are required, increasing expense and actually reducing reliability.
Other solutions relate to incorporating monitoring circuitry on the MEMS chip itself. The disadvantage here is that the reliability of the connection verification process is then no greater than the reliability of the component whose failure is to be monitored.
What is needed is an optical cross-connection verification technique that achieves high reliability with minimal cost, and that does not compromise the performance of the optical cross-connect system.
Systems and methods for reliable and low cost optical connection verification are provided by virtue of one embodiment of the present invention. One application is verification and monitoring of optical cross-connect performance. A connection is verified by splitting off optical signals at both ports of the connection, converting the signals to electrical signals, and then cross-correlating the electrical signals to verify connectivity between the ports. Lowpass filtering applied to the electrical signals may be applied to reduce the complexity of the cross-correlation.
According to a first aspect of the present invention, a method for verifying an optical connection between a first port and a second port includes: converting a portion of an optical signal at the first port to a first electrical signal, converting a portion of an optical signal at the second port to a second electrical signal, and generating a cross-correlation based on the first electrical signal and the second electrical signal.
According to a second aspect of the present invention, apparatus for verifying an optical connection between a first port and a second port includes: a first input accepting a first electrical signal derived by detection of an optical signal obtained from the first port, a second input accepting a second electrical signal derived by detection of an optical signal obtained from the second port, and a cross-correlation block that generates a cross-correlation signal based on the first electrical signal and the second electrical signal.
According to a third aspect of the present invention, a system for selectively cross-connecting optical lines includes: a plurality of optical input ports, a plurality of optical output ports, an optical switching fabric responsive to a control signal specifying a selected input port of the plurality of input ports to be connected to a selected output port of the plurality of output ports. The optical switching fabric provides a purely optical connection between the selected input port and the selected output port. The system further includes a first detector that converts a portion of an optical signal at the selected input port to a first electrical signal, a second detector that converts a portion of an optical signal at the selected output port to a second electrical signal, and a cross-correlation block that generates a cross-correlation signal based on the first electrical signal and the second electrical signal.
Further understanding of the nature and advantages of the inventions herein may be realized by reference to the remaining portions of the specification and the attached drawings.